# Engineering Physics Questions and Answers – Gases and Reactions

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This set of Engineering Physics Multiple Choice Questions & Answers (MCQs) focuses on “Gases and Reactions”.

1. In a vessel, the gas at a pressure P. If the mass of all molecules is halved and their speed is doubled, then the resultant pressure will be ___________
a) 4P
b) 2P
c) P
d) P/2

Explanation: According to the kinetic theory of gases, the pressure exerted by a gas,
P=1/3×mn/V v2
P/P=2
P=2P.

2. The absolute zero is the temperature, at which?
a) All substances exist in a solid state
b) Water freezes
c) Molecular motion ceases
d) Solid melts

Explanation: The absolute zero is the temperature at which all molecular motion ceases.

3. When we heat a gas-sample from 27°C to 327°C, then the initial average kinetic energy of the molecules was E. What will b the average kinetic energy?
a) 327E
b) 300E
c) 2E
d) √2E

Explanation: E/E=(273+327)/(273+27)=600/300
E=2E.

4. Temperature less than absolute zero is not possible.
a) True
b) False

Explanation: According to the kinetic interpretation of temperature, absolute temperature is proportional to the average kinetic energy of molecules. As the heat is removed, the temperature falls and velocity of molecules decreases. At absolute zero, the kinetic energy becomes zero. As the kinetic energy cannot be negative so no further decrease in kinetic energy is possible. Hence temperature cannot be decreased below absolute zero.

5. A given quantity of an ideal gas at pressure P and absolute temperature T. The isothermal bulk modulus of the gas is ___________
a) 2/3 P
b) P
c) 3/2 P
d) 2P

Explanation: At a given temperature,
P∆V+V∆P=0
P=V∆P/∆V=(∆P/∆V)/V=Isothermal bulk modulus.

6. Statement: The ration Cp/Cv is more for helium gas than that for hydrogen gas.
Reason: Atomic mass of helium is more than that of hydrogen.
a) Both statement and reason are true and the reason is the correct explanation of the statement
b) Both statement and reason are true but the reason is not a correct explanation of the statement
c) Statement it true but the reason is false
d) Both statement and reason are false

Explanation: Both the statement and reason are true but the reason is not a correct explanation of the statement
For monoatomic He,
γ=Cp/Cv = 5/3
For diatomic H2,
γ=Cp/Cv = 7/5.

7. The relation PV=RT can describe behaviour of the real gas at ___________
a) High temperature and high density
b) High temperature and low density
c) Low temperature and low density
d) Low temperature and high density

Explanation: At high temperature and low density, real gas behaves like an ideal gas. Then the intermolecular attractions and actual volume of gas molecules become negligible.

8. An absolute zero is the temperature at which?
a) Water solidifies
b) All gases become liquid
c) rms velocity becomes zero
d) Solids melt

Explanation: According to the kinetic theory of gases, absolute zero is that temperature at which all molecular motion ceases. Hence at T=0K, rms velocity becomes zero.

9. The temperature of a given mass is increased from 27°C to 327°C. The rms velocity of the molecules increases ___________
a) √2 times
b) 2 times
c) 2√2 times
d) 4 times

Explanation: vrms is proportional √T
v2/v1 =√(T2/T1)=√((273+327)/(273+27))=√(600/300)=√2
v2=√2 v1.

10. The ratio of the vapour densities of two gases at a given temperature is 9:8. The ratio of the rms velocities of their molecules is ___________
a) 3:2√2
b) 2√2:3
c) 9:8
d) 8:9

Explanation: At a given temperature,
(vrms (1))/(vrms (2))=√(8/9)=2√2:3.

Sanfoundry Global Education & Learning Series – Engineering Physics.

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